According to the method taught in the book, the tip of the tongue is slightly rolled up, pressed against the gums of the upper teeth, exhaled, and the tip of the tongue is shaken by the airflow.

It can't be soon. Don't be impatient with this. Take your time. Only by practicing every day can you practice well Otherwise, it will be difficult for you to change after practicing a semi-finished product!

My personal experience: the method of filling water is unscientific, and it is easy to choke, and I will accidentally practice my uvula, or my pronunciation is backward, and my speech is very ugly! When you start practicing, you can look up, move up with the airflow, and exhale a lot, but you should pay attention to the fact that it is easy to lack oxygen during rest. Fast people can pronounce pure tones in about three months, and slow for nearly a year. The only thing to pay attention to is to practice slowly and seriously, and never take shortcuts (there are no shortcuts). Don't be impatient. Time and energy will be used, and it will come out naturally!

Good luck!